Fuel cell-powered vehicles are being developed by the domestic and foreign automotive industry as a more fuel-efficient and less polluting alternative to the current internal combustion engines. Fuel cells, which produce energy through an electrochemical reaction, operate on hydrogen and oxygen gas. Currently, there is no infrastructure for wide-scale, high-volume production and distribution of hydrogen gas for transportation applications. Furthermore, storing pure hydrogen gas on-board a vehicle is not yet accepted by the general public; and the hydrogen storage methods currently available do not have sufficient capacity. An on-board “fuel processor”, which generates hydrogen from the liquid gasoline already used to fuel vehicles, is the most acceptable way to provide hydrogen for a fuel cell-powered vehicle.
Fuel processor technology is based mainly on large-scale industrial reactors that convert hydrocarbons into hydrogen and carbon oxides (synthesis gas) for the production of ammonia, methanol, and other commercially important chemicals or feedstock. These large-scale industrial reactors are operated continuously in a well-controlled, steady state manner. In contrast, fuel cells are often employed in small-scale, intermittent use applications, so the fuel processor for a fuel cell needs to be compact and respond quickly to changes in operating conditions. See U.S. Pat. No. 5,248,566 issued Sep. 28, 1993 to Kumar et al., the disclosure of which is incorporated by reference, for a general discussion of the use of a fuel cell in a vehicle. Autothermal reformers have the advantages of conservation of thermal energy and being compact, an important consideration for small passenger vehicles. Autothermal reformers operate at high temperatures (500-1000° C.) and use steam and air, or an oxygen containing gas, to convert the hydrocarbon fuel into hydrogen-rich gas. Like most other fuel processors, autothermal reformers rely on catalysts to obtain the highest concentrations of hydrogen in the shortest amount of time. To function well, autothermal reforming catalysts must adsorb and dissociate the hydrocarbon molecule and the water molecule. Thus, it is highly desirable to provide a catalyst that is active for the autothermal reforming of liquid fuels, such as diesel fuel via its ability to adsorb and dissociate both water and hydrocarbon molecules. It is also desirable to provide an autothermal reforming catalyst that does not contain expensive or rare precious metals so that fuel cell-powered vehicles can be affordable for the general public.
Passenger and heavy duty vehicles are becoming an attractive application for fuel cell systems. Replacing the internal combustion engine with a fuel cell system is a goal of a Department of Energy program and is an active R&D area for many automotive companies. A related, but different application is an “auxiliary power unit” (APU) for heavy duty and perhaps light duty vehicles. Such APUs would generate electricity with a fuel cell for all the electric auxiliaries of the vehicle which otherwise need to be powered by the engine. Examples include, but are not limited to, the water and fuel pumps, electronic devices and displays, living quarter power supply, and on-board air conditioner. Thus the engine can be shut off in stop-and-go traffic or during overnight parking at truck stops and campgrounds without interrupting the on-board electric power supply.
To operate the fuel cell, the hydrocarbon fuels, such as gasoline and diesel, must be first reformed into a gas mixture containing hydrogen and carbon oxides. Such a mixture is also known as reformate. The reforming process is generally carried out inside of a catalytic reactor, also known as reformer. The catalytic reactions inside of the reformer break down the chemical bonds of the hydrocarbon fuel and form H2, CO, CO2, simultaneously. There are several methods of catalytic reforming including partial oxidation (POX), steam reforming (SR), and autothermal reforming (ATR). ATR is generally considered the most efficient reforming process. The chemical reactions occurring in the autothermal reactors are facilitated by a catalyst that typically consist of a small amount of noble metals, such as Rh and Pt, resting on a substrate of alumina or ceria. Since the material costs of the noble metals are usually substantially higher than the other components in the catalyst, the amount of noble metal usage usually dominates the total cost of the catalyst. Therefore, to bring down the total cost of a fuel cell system, it is highly desirable to reduce or eliminate the noble metal usage in the catalytic reformer.
Since heavy duty vehicles operate on diesel fuel, the operators of these vehicles prefer that the APU operate on the same fuel. However, diesel fuel is difficult to convert to a hydrogen-rich gas for the fuel cells. Diesel fuel is a mixture of many different hydrocarbon species including paraffins, olefins, cycloparaffins, mono- and multi-ring aromatic compounds. The aromatic compounds are particularly difficult to reform into a hydrogen-rich reformate. They form carbonaceous deposits (coke) easily through the interaction with the catalyst surface and reduce the catalytic efficiency by blocking the passage between gas phase molecules and the active site. It is therefore desirable to have a catalyst with minimum propensity to form carbon deposits on the surface.
Operating the reformer at a higher reaction temperature will minimize the coke formation over the catalyst surface. However, higher reaction temperature results in catalyst structural change which could also lead to the loss of the catalyst activity. Even at temperatures in the range of 700-800° C., which are typical for ATR reactors, the aforementioned catalyst can loose 50% of their activity over a 1000 hour period. This deactivation is caused by the small noble metal particles consolidating into larger ones, resulting in a decrease of active catalytic surface area, and by some loss of noble metal by evaporation. Furthermore, higher operating temperature could also lead to the deterioration of the catalyst support, such as the reduction of surface area and loss of porosity. The deactivation becomes more severe at higher temperatures.
Another challenge in diesel fuel reforming is sulfur poisoning of the catalyst. There is generally a significantly higher level of sulfur content in heavy hydrocarbon fuels, such as diesel Sulfur interacts with most metallic catalysts by chemically absorbing on the metal surface and blocking the access of reactant to the active sites. It is desirable to have a reforming catalyst that is tolerant to the presence of sulfur in the hydrocarbon fuels.
It has been previously established that certain perovskites function surprisingly well at autothermal reforming catalysts. Perovskite oxides have the general formula ABO3. The A cation is bigger than the B cation in accordance with Goldschmidt's empirical relationship0.75<(rA+rO)/√2(rB+rO)<1.0where rA is the radius of the A cation, rB is the radius of the B cation, and rO is the radius of the oxygen anion. The perovskite structure will tolerate partial substitution for both the A and B cations, which allows the properties of the compounds to be modified. There are many perovskite related structures: the Ruddlesden-Popper structure ((AO)-(ABO3)n, n=1,2, . . . ) is a perovskite layered with a rock salt, the tungsten bronze structure (A0.6BO3) is a defective perovskite with A-site vacancies, and the Brown-Millerite structure (A2B2O5) is defective perovskite with oxygen vacancies. The perovskite oxides, and the related structures, with La, Y, rare earths, and/or alkaline earths on the A site and transition metals on the B site have in many cases been found to be good electronic and/or good ionic conductors.
More specifically it has been found that mixed oxides of La with Cr or Al of the general formula LaCrO3 or LaAlO3 are stable in both air and in hydrogen-rich gas at elevated temperature. When doped with certain other elements, these perovskites are good catalysts for oxidizing and steam-reforming hydrocarbons. For example, a material of the general composition LaAl0.9Ni0.1O0.3 was found to yield 12 moles of hydrogen per mole of iso-octane when iso-octane is mixed with a substoichiometric amount of air and some steam and then passed over the catalyst. There are, however, certain disadvantages associated with these catalysts. First of all, it was found that these catalysts produce lower hydrogen yield and poor efficiency during reforming of diesel like fuel as the result of high catalytic light-off temperature and low hydrocarbon conversion. It is also known that Ni containing catalysts are intolerant to the low sulfur content in the fuel and coke is readily formed during the reforming reaction.
It is therefore desirable to have a reforming catalyst that is stable under high operating temperature and provides overall lower costs.